tile with not so good lighting.

why don't you post a pic of your bathroom? all of your questions depend on specifics. without seeing the bathroom, the layout, the tub, the lighting, etc,, can't answer them. dark tile doesn't necessarily make a room smaller or darker. You can't use large tiles (usually) on a shower floor (again, it depends on the slope and makeup of the shower), you can put the tile behind the tub if you think that area will get splashed with water.

the choices you have are fine. use the mosaic for the shower floor and any accents (niche). large white tile for walls and the darker one for the floors outside the shower. It won't make the room look too small.

I didn't post a picture of my current bathroom because it is very small, dark, we are losing one bedroom to make the bathroom bigger. Thanks for your response